Toner is a developer material for a laser printer or a copy. The operation of a printer is substantially as follows: after exposure, an electrostatic latent image can be formed on the surface of a photosensitive drum charged uniformly. The toner charged by a developing roller and a blade can develop the electrostatic latent image on the surface of the photosensitive drum to form an image. Attracted by opposite charges on the surface of a paper, the image on the surface of photosensitive drum can be transferred and printed on the paper. Then we will get the desired printed image fixed by a heating roller and a pressure roller.
It can be seen from the printing process described above that the toner is charged by the developer roller and the blade, and the charges are obtained by the surface of the toner. The charged toner can develop on the surface of the photosensitive drum to form a pattern to be printed. In order to enable the toner to have good chargeability, a charge control agent (CCA) is required to be dispersed on the surface of the toner particles as uniformly as possible. Before being transferred and printed from the photosensitive drum to the paper, the toner is required to hold charged state continuously. If the charging characteristic and charge retention capacity of the toner is not good, it is difficult to develop or transfer print. It is also difficult to get the image with desired printing effect.
Similarly, the toner should have a number of other properties, such as excellent transfer printing property, low-temperature fixation and storage stability. The toner with a good spherical shape has a high transfer printing efficiency, which can be easily transferred and printed from the photosensitive drum to the paper, while reducing or preventing toner remained on the surface of the photosensitive drum. To reduce the environmental impact, the toner should fix on the paper at a temperature as low as possible. At the same time, the toner should have a good storage stability without melt adhesion even exposed to an high temperature condition during being handled or transported. The toner of core-shell structure improves its fixation property by a core with a low glass-transition temperature (low Tg), and enhances blocking resistance by a shell with a high glass-transition temperature (high Tg), By employing such a layer structure, it is possible to meet requirements of both low-temperature fixation and storage stability.
Conventional mechanical pulverizing toner can be obtained by melting and blending resin, CCA, pigment and wax, then pulverizing and classifying. Since mechanical pulverizing toner has an irregular shape, the pressure applied by the blade to each toner particles varies, and CCA tends to form a phase separated from a binder resin. CCA particles exposed on the surface of the toner can easily fall off. Therefore, mechanical pulverizing toner is non-uniformly charged and has a very wide charges distribution. Chinese Patent CN101427186 provides a mechanical pulverizing toner based on styrene/acrylate and polyester binder resin, using a charge control agent of styrene/acrylate which has a good compatibility with the toner. Therefore, distribution uniformity and retention capacity of charges of the toner has been greatly improved. But it is difficult to meet the requirements of low-temperature fixation. Further, the process in which the toner particles are collided with one another in 5 to 20 minutes for purpose of spheronization at 75 to 100 m/s linear velocity consumes large energy and has high requirement for equipments. Furthermore, it is difficult to get a positive spherical toner.
Compared with traditional mechanical pulverizing toners, the conventional suspension polymerization toner of core-shell structure with dense charges can be obtained by dispersing CCA and waxes, pigments and other components together into a monomer uniformly, and carrying out polymerization after high-speed shearing and granulation. CCA in the toner particles uniformly disperse, and the toner particles have a good sphericity. Thereby, it has a good uniformity of charge distribution and transfer printing efficiency. But the method can result in a significant amount of CCA distributed in the central region of the toner particles. Charges of the toner can be generated by the friction between the blade and the CCA on the surface of the toner. However the CCA dispersed in the central region of toner particles does not involve in such a friction, it can result in an inefficient use of the CCA. Therefore, there is a need to develop an effective method for increasing the charge density on the surface of the toner.
Chinese Patent CN101473274 provides a toner of core-shell structure which comprises a rigid and thin shell layer with dense CCA distributing formed by coating the surface of the toner core particles which is prepared by suspension polymerization process with CCA, using a polymer formed by polymerization reaction of the shell monomer components. The method demands the particle diameter distribution of CCA being used be tightly controlled. It results in a large number of CCA dissociating because a poor compatibility of CCA and shell resin.
Another Chinese patent CN1707366 provides a toner of core-shell structure with positive charges which comprises a surface layer with dense charges having a thickness of about 20-200 nm formed by polymerizing a monomer containing amine or ammonium salt on the surface of the toner core particles, or salting out, melting and bonding with a polymer particle containing amine or ammonium salt. However, it is not easy to obtain a toner particle with positive spherical shape in this way.